Recently, the concept of the wireless sensor network (WSN) has received considerable attention. A WSN typically includes a collection of low-power transceivers (henceforth called sensors) each having some kind of sensor function for one or more properties of an environment in which they are placed. The term “environment” here has a very broad meaning and could include, for example:—a geographical area such as a farmer's field, an area of ground requiring monitoring for security reasons, or a volcano; a specific facility such as an industrial plant, a hospital, a retail store or financial institution; or a human body. Likewise, the range of properties which might be sensed is wide, including temperature, pressure, sound, vibration, motion, the presence of specific chemicals, etc.
Each sensor is capable of transmitting sensor data, usually as discrete packets, to any other devices in its vicinity, usually to another sensor. By relaying data from one sensor to another, the sensed data can be directed to a so-called sink or base station and gathered (temporarily stored). Although the precise communication standard used by the sensors is not important, one suitable standard is IEEE802.15.4, a current implementation of which is called ZigBee.
Depending upon the capabilities of the sink, the data can be forwarded from the sink directly or indirectly to some form of outside entity, typically via another network such as a mobile telephone network or the Internet. Where the sink is able to communicate with another network it can also be called a gateway (GW).
In some implementations, the terms sink, base station and gateway mean the same thing; in others they denote distinct functions, in which case the sink will communicate the gathered data to a separate base station and/or gateway for further transmission, possibly after some kind of aggregation or other processing.
Moreover, in some implementations, the sensors (or a subset thereof) are also capable of acting as the sink. Multiple sinks, and multiple gateways, may be present in a WSN but for simplicity, a single sink is assumed in the following description.
In the present specification, the terms “sink” and “base station” are used synonymously to denote any kind of data-gathering entity in a wireless sensor network whether or not it also acts as a gateway.
Some possible applications of WSNs are shown in FIG. 1. A WSN applied to the human body is called a Body Area Network (BAN), as indicated at 10 in the upper part of FIG. 1. In this instance, the sensors 12 might monitor body functions such as heartbeat and blood pressure, and transmit their data to a sink 14 in the form of a portable computing apparatus such as a mobile phone, PC or PDA. As indicated this would normally have a wireless link, via another network 50, to an external data server 16 for analysis and forwarding on, if necessary, to a data centre (“SBS Platform”) 18, allowing decisions to be taken based on the sensed data. For example, changes in the heartbeat of a hospital patient might lead to a decision to signal medical staff to attend to the patient.
The left-hand lower part of FIG. 1 depicts a WSN 20 applied to a geographical area, for example to monitor environmental conditions such as air quality. Such a WSN is also termed an Environment Sensor Network or ESN. By being scattered over a geographical area, the sensors 22 are essentially fixed in this application. As indicated, the sensors might communicate using the above-mentioned Zigbee standard with the data being routed to a gateway GW 24 for further transmission over network 50.
Next to this in FIG. 1 is indicated another form of WSN 30 in which the nodes are sensors on board vehicles 32, and are thus mobile. In this case the sink is provided in the form of a gateway 34 which might be fixed to a mast at a traffic intersection for example, or might itself be mobile by mounting it on another vehicle. Again, monitoring of pollution is one possible application. Although not shown in FIG. 1, each individual vehicle 32 may also have its own WSN formed by sensors at various points in and on the vehicle, for monitoring parameters such as speed, temperature, tyre pressure and so forth. Such a WSN is an example of an Object Sensor Network or OSN.
The lower right-hand part of the Figure indicates a WSN 40 for assisting with disaster prediction, recovery, or prevention. As before, sensors 42 are scattered around a geographical area to be monitored, with a gateway 44 acting as the sink for receiving the sensor data and forwarding the same over network 50 to server 16. By raising alarms in response to sensor data from buildings, the ground or the atmosphere, rescue operations can be started more quickly to deal with earthquakes, fire or flooding. Compared to conventional monitoring networks, WSNs are cheaper to deploy and at the same time they provide more powerful and accurate real-time tools to acquire the data.
As will be apparent from FIG. 1, in general the sensors of a wireless sensor network may be fixed or mobile, and the sink may be fixed or mobile. However, the present invention concerns a WSN in which the sink is mobile. In general, we can assume that the sink is movable on demand, but otherwise stationary.
Commonly, the sensors are unattended devices of low computational ability and reliant on battery power; thus, power consumption of sensors is a major consideration. Transmission of data is typically the most power-hungry function of a sensor. For this reason, it is preferable for a sensor to communicate only with its nearest neighbours, necessitating the use of multi-hop techniques to enable data to reach the sink by several different routes. Another technique employed to conserve battery power is to deactivate sensors which are not currently engaged in sensing or communication (including relaying). Thus, sensors may alternate between active and inactive states (also called “awake” and “asleep”), for example in response to the presence or absence of a sensed property or incoming data. In this way the useful lifetime of the sensor can be prolonged. However, unless a sensor has some way to replenish its power, its battery will eventually become exhausted, at which point it assumes a “dead” state. Dead sensors reduce the coverage of the network and restrict the number of available routes for data, to the point where in the worst case, the WSN is no longer operable. Consequently, related to the need to conserve battery power of sensors is the desire to keep each sensor “alive” for as long as possible. This is particularly challenging when the sensors are moving, for example as a result of being mounted on a vehicle or a human body.
As will be apparent from the above discussion, it is possible to define one of a limited number of states for each sensor at a given point in time. The sensor may be “active”, in the sense of transmitting its own sensed data; it may be acting as a relay (this is distinguished from “active” for present purposes); it may be “inactive” due to not having any data to transmit or relay; or it may be dead. The concept of the “state” of a sensor is important for managing the network, as explained in more detail below.
Another consideration, of particular relevance to the present invention, is appropriate positioning of the sink. Generally, the sensors transmit data in all directions indiscriminately without knowing or caring which other nodes receive it. A sink far from the more active part(s) of a wireless sensor network will tend to receive less data, with greater delay (latency), and incur more power expenditure by the sensors, than one placed closer to the action. In a sparse WSN (one having relatively few sensors for the geographical area covered), some positions of the sink may not allow the sink to communicate with all parts of the WSN. Conversely, in a dense WSN there is generally no problem for all sensors to reach the sink, but those sensors closest to the sink will tend to suffer high power drain owing to the large demands on them for relaying sensor data to the sink. This will tend to drain the available power in a short time if the sink stays still.
Thus, it is unlikely that a fixed sink will remain optimally positioned for any length of time. By its nature, a wireless sensor network has a constantly-changing configuration, owing to changes of state of the sensors, their movements if any, and changes in the property or properties being sensed, so the appropriate position for the sink is liable to change frequently, possibly over quite short timescales.
In one form of wireless sensor network, the sensors are RFID-based devices which might not be reliant on a battery power, but as the available transmission power of such devices is very low, similar considerations still apply regarding placement of the sink.
Thus, dynamic repositioning of the sink in a wireless sensor network has been proposed as a technique for increasing sensor lifetime whilst improving the quality and throughput of communications over the WSN while reducing the potential delays.
Unfortunately, it has been shown that the problem of sink positioning in a WSN is an “NP-complete” problem and thus difficult to solve with the very limited computing resources available in the WSN.
Accordingly, it is desirable to find a solution for sink positioning in a WSN which is efficient in terms of computing resources and effective to position the sink at desirable locations.
It is further desirable to provide a technique for sink positioning which takes account of the needs of the sensors in terms of their remaining energy stores.